This invention relates to an inspection method and an inspection apparatus using nuclear magnetic resonance for measuring a map of diffusion coefficients or a map of signal intensity emphasizing signal attenuation due to diffusion. More particularly, this invention relates to an inspection method and an inspection apparatus which are referred to as "diffusion spectroscopic imaging", separate materials contained in an inspection object in accordance with molecules, and acquire a map of their diffusion coefficients or a map emphasizing signal attenuation due to diffusion.
Various methods of measuring diffusion coefficients of an inspection object or apparent diffusion coefficients of a living body due to perfusion have been proposed in nuclear magnetic resonance inspection apparatuses. The methods which have been employed most widely at present are based on a pulse sequence of Stejskal-Tanner (E. O. Stejskal and J. E. Tanner, The Journal of Chemical Physics, No. 42, pp. 288-292 (1965)).
To measure the diffusion coefficient, etc, this method first excites a nuclear spin by a radio frequency magnetic field and then applies at least two gradient magnetic fields compensating for one another to acquire signals. Here, the term "compensating for one another" means that influences which rotate the phase of the nuclear spin are cancelled if molecules do not move.
In other words, if diffusion exists, the influences of phase rotation cannot be completely cancelled and a signal intensity attenuates at a proportion corresponding to the application amplitude and time of the gradient magnetic field. Therefore, measurement is conducted a plurality of times by changing the application amplitude and time of the gradient magnetic field and the diffusion coefficient can be determined from the attenuation rate of the signal intensity. The gradient magnetic field applied for measuring the diffusion coefficient is referred to as "MPG (Motion Probing Gradient)".
Numeric representation of the influences of the gradient magnetic field on the attenuation rate of the signal intensity is referred to as a "b-factor (gradient factor)".
A method of imaging the diffusion coefficient by extending this method is reported by D. LeBihan et al in Radiology, No. 161, pp. 401-407 (1986). This method combines a gradient magnetic field applied for imaging with an MPG, shoots a plurality of images by changing b-factor and calculates the diffusion coefficient at a given pixel from the attenuation rate of the signal intensity of each image at the corresponding pixel.
As another extension of the method described above, a method of measuring a diffusion coefficient for each molecule (metabolites) contained in a living body, e.g. a diffusion coefficient of N-acetylasparate, adenasine triphosphate, or the like, formed by the molecules (diffusion spectroscopy) is reported by C. T. W. Moonen et al in Magnetic Resonance in Medicine, No. 13, pp. 467-477 (1990). This method separates each molecule by utilizing the difference of a nuclear magnetic resonance frequency, which is slightly change from molecule to molecule, (i.e. chemical shift), conducts measurement a plurality of times by changing the application amplitude of the MPG, and calculates the diffusion coefficient of the molecule from the attenuation rate of the signal intensity of each molecule.
In connection with a method of measuring the map of the diffusion coefficient for each molecule contained in the living body (diffusion spectroscopic imaging), the closest related art is reported by M. Xue et al in Proceedings of the Society of Magnetic Resonance in Medicine, Twelfth Meeting, pp. 68 (1993). This method sets the application amplitude of the MPG to a certain high level and conducts once imaging for each molecule (diffusion weighted spectroscopic imaging). Though not capable of calculating the diffusion coefficient, this method can measure a spectroscopic image of a diffusion emphasis type in which a signal intensity strongly attenuates at a position at which diffusion is vigorous. The method employs 3D-CSI (Chemical Shift Imaging) which has been used most widely at present as the spectroscopic imaging method.
As related references to the present invention relating to spectroscoping imaging, mention can be made of JP-A-59-90552 and JP-A-61-13143. They are the prior art methods which execute high speed spectroscopic imaging. Other prior art references relating to the measurement of the diffusion coefficient include JP-A-4-135538 and JP-A-4-357934.